Chlorine-Induced High-Temperature Corrosion of Boiler Steels

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Chlorine-induced high temperature corrosion of boiler steels combusting Sha Erhu coal compared with biomass Yacheng Liu, Weidong Fan, Xiaofeng Wu, and Xiang Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03143 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Chlorine-induced high temperature corrosion of boiler steels combusting Sha Erhu coal compared with biomass Yacheng Liu1, Weidong Fan∗1, Xiaofeng Wu1, Xiang Zhang2 1. School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2. Shanghai Boiler Works, Ltd, Shanghai 200245, China

ABSTRACT: Chlorine, as the harmful element with high volatility and reactivity existing in certain coal and biofuels, mainly contributes to severe deposit formation and high temperature corrosion. Sha Erhu (SEH) coal is characterized with high amounts of alkali metal (mainly sodium, Na) and chlorine (Cl) and relatively less amounts of sulphur (S), which is close to the characteristics of biomass. However, the proportion of ash and mineral in most biomass is far lower than that in coal, while the volatile and potassium (K) content is typically higher. In light of these distinct corrosive elements, comparative investigation of the high-temperature corrosion characteristics of boiler steels (T91, 12Cr1MoVG, and TP347H) firing high- or low-chlorine coal and biomass is meaningful. A series of 168 h corrosion experiments involving SEH ash, Da Nahu (DNH) ash, straw ash, wood chip ash, and their mixtures were performed on the tube furnace with the designing a new type of double-tube corrosion probe to consider temperature gradient from flue gas temperature to tube wall temperature. The results show that both SEH ash and straw ash deposits cause severe material degradation via higher contents of Cl and alkali, but the corrosion mechanism of straw ash can be inferred by gaseous KCl-induced active oxidation, and for SEH ash, accelerated corrosion is the complicated molten corrosion, involving electrochemical attack and chemical corrosion. In

∗

Corresponding author. Tel.: +86-21-34208287. Fax: +86-21-34206115. E-mail address: [email protected]. 1

ACS Paragon Plus Environment

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comparison with high-chlorine fuel, the corrosivity of DNH ash and wood chip ash on the specimen is weak. Moreover, the austenitic steel TP347H has higher corrosion resistance than other types because of its higher contents of chromium and nickel. The experimental results also show that a mixture of DNH ash and straw ash can alleviate corrosion via the formation of silicate or aluminium-silicate. However, a mixture of SEH ash and wood chip ash results in severe corrosion at the mass ratio of 2 to 1.

Keywords: high temperature corrosion; biomass; boiler steel; low melting eutectics; active oxidation

1. INTRODUCTION Recently, vast storage of the Sha ErHu (SEH) coalfield characterized by high alkali and high chlorine (exceeding 0.3 wt.% on a dry-coal basis) was explored in XinJiang District, China. Ash-related problems, such as severe fouling, slagging, agglomeration, and corrosion, are the most intractable issues in large-scale pulverized coal (PC) or circulating-fluidized-bed (CFB) utility boiler-firing SEH coal. Alkali- and chlorine-induced rapid build-up of unmanageable deposits on the steel tube surfaces decreases the heat transfer coefficient and reduces the boiler efficiency. Furthermore, accumulated ash with a high chlorine concentration on metal tube surfaces may lead to catastrophic high temperature corrosion.1 The behaviour of Cl in a coal combustion is schematically shown in Figure 1. Inorganic chlorides and organic chlorides2 are released into the flue gas mainly containing Cl2(g), HCl(g), NaCl(g), and KCl(g), which can cause direct corrosion by reacting with protective oxide scale. In addition, the alkali chlorides can condense on the boiler tube surface or ash particle during their flow in the horizontal flue. Biomass, as a renewable and carbon-neutral energy source, produces less toxic emission than fossil or waste fuels, which is beneficial to environmental protection and conforms to social development.3 A report shows that the biomass-power installed 2


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capacity will reach to 30 GW by 2020 in China.4 However, many issues hinder the efficient and clean utilization of biomass in the rapid development of biomass-firing power boilers. For example, high-temperature chlorine corrosion of superheaters or reheaters is generally considered in boilers combusting biomass or municipal wastes rich in alkali metals and chlorine. Although both SEH coal and biomass have high Cl and alkali metal contents, SEH coal has higher contents of Na, Al, Si, and S in the combustion process, while biomass has a higher value of K. NaCl(g) is considered as the most stable alkali-metal-containing substance in the flue gas and the dominant substance that influences SEH coal high-temperature corrosion, while for biomass, KCl(g) is reported as playing a similar role.4 Enestam et al.5 investigated the corrosivities of pure NaCl and KCl on superheater tubes, and their experimental data showed that NaCl has a similar corrosive behaviour to KCl from the practical point of view, but the discrepancies may be significant when referring to detailed corrosion mechanisms.

The prevailing Cl-related mechanisms of high-temperature corrosion involve active oxidation and fluxing caused by molten alkali salts.6 The molten salts have the ability to dissolve off the protective oxide scale, exposing the alloy substrate and making it vulnerable to attack. The high-temperature molten corrosion was ascertained primarily from alkali chlorides, NaCl and KCl. NaCl and KCl have melting points of 801 and 774

,6 respectively, but the formation of eutectic

mixtures with FeCl2, Na2CrO4, Na2SO4 and other depositing or corrosion products dramatically lowers their melting points; for example, the melting temperature of NaCl-Na2CrO4 is 557

.7 The

main method to alleviate high-temperature corrosion in the boiler firing biomass is to limit the final steam temperature5 and, hence, decrease the boiler’s efficiency. However, economic fuel utilization can be improved by increasing the final steam parameters to improve the electrical efficiency. To 3


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guarantee appropriate high steam parameters for utility boiler-firing SEH coal, potential countermeasures have been proposed, including co-firing, additives, leaching, and alloying.6 An extensive technology of superalloys and coatings have been developed in an endeavour to minimize the corrosion attack. The anti-corrosion element Cr in the alloy was reported to be unreasonable for alleviating Cl-induced high temperature corrosion.8 Some additives, such as tungsten salts9 or molybdenum salts,10 have been studied to mitigate the corrosion problem by preventing the formation of alkali chlorides on the boiler tube surfaces, and the alkali element is captured by tungsten or molybdenum to form polytungstates or molybdates. However, the kinetics of alkali-containing species on the transformation and sequestration has not been given considerable attention. From the practical application in the utility boiler, co-firing of the reasonable fuel is an effective and economical method to alleviate corrosion. Moreover, co-firing of reasonable coal and biomass is a promising approach to increase the utilization of biofuel and decrease CO2 emission.11,12 In this paper, a comparative evaluation was conducted on the high-temperature corrosion characteristics of boiler steels (T91, 12Cr1MoVG, and TP347H) firing high- or low-chlorine coal and biomass. Laboratory-scale corrosion experiments involving SEH ash, Da Nahu (DNH) ash, straw ash, wood chip ash, and their mixtures were performed on the tube furnace with the use of a newly designed type of double-tube corrosion probe to regulate two different temperatures: flue gas temperature and tube wall temperature, while similar experiments13,

14

were performed under quasi

isothermal condition. With the aid of a scanning electronic microscope coupled with an energy dispersive spectrometer (SEM-EDS) and poly-functional X-ray diffractometer (XRD), the appearance and microstructure, the elemental concentration distribution, and the composition of corrosion products on the specimens after corrosion were analysed. The objective is to investigate 4


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the Cl-induced high temperature corrosion mechanism on boiler steels beneath various coal ash deposits, biomass ash deposits, or blended ash deposits of coal and biomass.

2. EXPERIMENTAL Laboratory-scale corrosion tests were conducted in a horizontal tube furnace (a thermal power of 5 kW) equipped with an alundum-lined tube fitted with a PID controller. As shown schematically in Figure 2, the experimental system adopted to simulate corrosion performance on reheaters in air-fired atmosphere is primarily comprised of three parts: preparation of the simulated flue gas, a sophisticated metal wall temperature control system, and exhaust gas treatment. The gaseous environment except H2O(g) was provided by a mixture of commercially available gas cylinders and was controlled by a mass flowmeter. Steam in the simulated flue gas was achieved by humidification in a constant temperature water bath. To obtain a steeper temperature gradient across the hot flue gas, the deposit layer, and the steel specimen conforming to actual boiler environments, an inner air-cooling metal tube, a protective alundum tube, and a filamentous thermocouple were employed. The temperatures of the simulated flue gas and steel specimen were controlled at 950

and 610

,

respectively, and then cooled internally by pressured air. The temperatures were monitored through two thermocouples inserted into the furnace and steel specimens. The necessities considering the temperature gradient on the steel specimen are listed as follows: (1) alkali metal chlorides in the ash deposit would enrich on the cooler surface of the steel specimen, the primary mechanisms are identified as evaporation-condensation and thermophoretic force,15 and this phenomenon was confirmed by the experimental result of Lindberg et al.,16 and (2) the detachment of the oxide layer may be easier because of the differences in the thermal expansion of the oxide and the base

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material.17 Further details regarding the laboratory-scale corrosion setup can be found in our previous research.18

Three commonly used boiler steel materials (martensitic steel T91, Pearlitic steel 12Cr1MoVG, and austenitic steel TP347H) were investigated in the high temperature corrosion test regarding their corrosion resistance. The main differences of three steel specimens on chemical composition are the contents of alloying elements Cr and Ni. Steel specimens with a size of 12 mm × 12 mm × 6 mm were cut from the original tubes, and then the final surface was polished with P#800 SIC-paper and cleaned with ethanol. To analyse the Cl-induced high temperature corrosion in coal and biomass, high- and low-chlorine coal and biomass ash were prepared. Because of the high content of alkali metal and chlorine, four different ash samples (SEH, DNH, straw, wood chip) were ashed at 500 in a muffle furnace for 72 h to avoid the volatilization of alkali and chlorine.19 Table 1 summarizes these four ash sample compositions by X-ray Fluorescence Spectrometer (XRF) analysis. It can be seen that SEH ash and straw ash have higher content of Cl than other ashes, and the alkali metal in the two biomasses is higher than that in coal, especially the element K. Moreover, the content of element S in the DNH ash is higher than that in other types, and many literature reports20,21 indicated that S could reduce Cl-induced corrosion by the formation of less corrosive alkali metal sulphate, of which the corrosivity can be effectively hindered by oxide scale Cr2O3. Co-firing of coal and biomass is a common and effective approach to alleviate high-temperature corrosion in the utility boiler, and considering the input energy density, the amount of biomass in the mixture is not high; thus, it is reasonable to adopt the weight ratio of coal ash and biomass ash as 2 to 1. In this paper, high Cl coal ash (SEH) is blended with low Cl biomass ash (wood chip) to simulate deposit on

6


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heating surface, and the mass ratio is 2 to 1. Moreover, low Cl coal ash (DNH) is blended with high Cl biomass ash (straw), and the ratio is also 2 to 1.

These ash deposits were mixed with ethanol to form thick slurries for painting onto the specimens to achieve the optimal simulation of ash deposit, and the experimental duration of each test was 168 h. The detailed information of the corrosion test is given in Table 2. After exposure, the coated ash was brushed, and then the mass change of each specimen was measured prior to and after exposure using a four-decimal analytical electronic balance (type: FA1104, accuracy: 0.1 mg). Next, the specimens were mounted in conductive resin, and then cross sections of the steel specimens were prepared for SEM-EDS (type: JEOL JSM-7800F Prime) characterization. The main crystalline compounds of the four ash deposits were identified by poly-functional X-ray powder diffractometer (XRD) using D8 ADVANCE Da Vinci with the characteristic Cu radiation and a scintillation detector. The crystalline compounds were identified by comparison with the standards in the Joint Committee of Powder Diffraction Standards.

3. RESULTS AND DISCUSSION 3.1 Cl-induced high-temperature corrosion by coal ash In China, the chlorine content in coal is generally low, and the content is primarily less than 0.05 wt.%, which is well below the world average and belongs to low chlorine coal.18 Thus, the main focus on high-temperature corrosion is on S-induced corrosion; however, from the XRF analysis result of SEH ash shown in Table 1, a high Cl content is found in the coal. Lehmusto et al.22 performed experiments involving the reaction of eight cationic chlorides with protective oxide Cr2O3 at various temperatures; their result showed that only alkali metal chlorides could destroy the

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protective scale; that is, the partitioning forms of element Cl in the ash deposit have a great impact on corrosion. The XRD analysis of the SEH and DNH coal ash is shown in Figure 3, and semi-quantitative analysis of these detected crystals is also presented. It can be seen that sticky silicates and sulphates of alkali and alkaline earth metal are the main composition in SEH coal ash; simultaneously, a small amount (0.53 wt.%) of NaCl is also detected. However, the main components of DNH coal ash are quartz and calcium salt. Moreover, alkali metal exists in the form of aluminosilicate. Through thermodynamic equilibrium calculations by many scholars,23,24 it can be concluded that alkali metal element binds with Si, S, Cl, and Al following the sequence of aluminosilicate, silicate, sulphate, and chloride. In addition, Cl preferentially forms NaCl rather than HCl during Cl release.18

The mass gain of each steel specimen can macroscopically represent the corrosion degree, despite some drawbacks, such as embedment of fine ash particles in the scale microstructure and spalling of oxides. After exposure time up to 168 h, the mass gain of the steel specimens corroded in various ash deposits, shown in Figure 4, reveals that low-chlorine coal ash has a lower value than high-chlorine coal ash, and the value of steel TP347H is less than 1 mg/cm2 beneath DNH coal ash. That is, austenitic steel TP347H exhibits better corrosion resistance under DNH ash deposit. However, SEH ash deposit produces a mass gain of approximately 7 mg/cm2, 21 mg/cm2, and 40 mg/cm2 with TP347H, T91, and 12Cr1MoVG, respectively. Thus, the corrosion resistance of three steel specimens is ranked as follows: austenitic steel TP347H, martensitic steel T91, and pearlitic steel 12Cr1MoVG. Moreover, the sequence represents the tested steel grades for their application in high-temperature heating tubes of utility boilers firing SEH coal that contains high chlorine and sodium. Although TP347H has higher contents of Cr and Ni than those of other types, severe 8


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corrosion also occurs because of the NaCl existing in the SEH coal ash. Moreover, NaCl will be rich in the deposit close to the metal surface because of the evaporation-condensation mechanism.16 The negative effect of NaCl on the protective oxide scale formed on the alloy surface can be concluded into two ways. First, gaseous NaCl could react with Cr2O3 to form unprotective alkali chromate, releasing Cl2 or HCl to accelerate substrate oxidation. However, reaction of solid NaCl with the protective Cr2O3 scale is not thermodynamically favoured, as verified by the thermo-gravimetric experiment of Lehmusto et al.25 Second, the formed alkali chromate Na2CrO4 can combine with NaCl to result in low temperature-eutectic, and the melting temperature of NaCl-Na2CrO4 is 557 ℃.7 That is, molten alkali chloride forms on the metal surface, and unfortunately, protective oxide scale Cr2O3 has a high solubility in molten NaCl.6 Thus, corrosion resistance is reduced as the Cr component dissolves in the molten chlorides, and the presence of a molten phase at the alloy/scale interface indicates that severe corrosion occurs.26

To explain these observations clearly, the microscopic characteristics of steel specimens were analysed with the aid of SEM observations, and elemental distribution maps were obtained by EDS analysis. Figure 5 displays the cross-sectional microstructure and elemental distribution maps results of the corrosion products of TP347H steel at 610 ℃ corroded in SEH coal ash deposit for 168 h. A large amount of corrosion product was not able to be brushed off because of the molten phase formed at the scale/substrate interface. Moreover, the corrosion rate along the metal surface is uneven, as seen from the elemental distribution maps that alloying elements Fe, Cr, and Ni are rare, whereas Na, Cl, and Si are of higher contents in the corrosion pit. It can be inferred that corrosive NaCl can react with the chromium/chromium oxide and iron/iron oxide layers to release molecular chlorine, which can diffuse through cracks and fissures in the oxide scale to reach the scale-metal 9


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interface and accelerate metal oxidation. It should be noted that the concentration distribution is based on the colour in EDS maps: the heavier the element is, the lighter the colour. Element Ca is found to be rich in the corrosion product, and its high concentration distribution is associated mainly with Si, which indicates the presence of CaSiO3. After destruction of the protective Cr2O3, element S can diffuse inward to combine with the alloying elements; that is, S is detected in the inner substrate. The Si distribution map shows that there exists some amount in the corrosion product, possibly caused by the formation of sticky alkali silicate. An enrichment of Ni can also be seen from the EDS maps in the outer layer of substrate. The high concentration count of nickel is not diffused from the inner alloy substrate but is the result of Cl-induced selective corrosion, depleting the substrate surface of most Fe and Cr and leaving almost solely Ni because of its lower affinity with Cl than others. The Gibbs free energy at 610

for formation of NiCl2 (-109 kJ/mol) is less negative than

FeCl2 (-184 kJ/mol) or CrCl2 (-172 kJ/mol); thus, Fe and Cr are more easily attacked than Ni. Figure 6 shows the cross-sectional microstructure of the specimen corroded in two coal ashes, and the thin compact oxide layer formed on the surface of TP347H corroded under DNH ash deposit. Enestam et al.5 has defined the uniform compact oxide layer as type 1 oxide, which provided a barrier for further diffusion of oxygen and other gaseous species into the metal; as a result, oxidation corrosion is limited. It can be inferred that the molten phase occurs at the substrate/scale interface for T91 corroded under SEH ash, consistent with the results of TP347H. However, no molten phase is detected for 12Cr1MoVG, and the oxide layer is easily detached from the substrate because 12Cr1MoVG is characterized with less Cr content compared to that of the others. Its iron oxide scale has much poorer protective properties, as it has higher vapour pressure and diffusion rate of FeCl2 compared to chromium-rich oxide.27

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3.2 Cl-induced high temperature corrosion by biomass ash It is promising to develop new technologies capable of combusting large amount of biomass in existing units, with high efficiency and high fuel flexibility. The key is to solve the problem of Cl-induced high temperature corrosion in utility boiler-firing high Cl biomass. The XRF analysis results shown in Table 1 reveal that straw ash is characterized by high Cl content (10.29 wt.%), which is close to that of the SEH ash (11.37 wt.%). However, the alkali metal (K+Na) in biomass ash is higher than in coal ash. The XRD analysis result of the straw and wood chip ash is shown in Figure 7, and semi-quantitative analysis of these detected crystals is also presented. It can be seen that a large proportion of KCl exists in straw ash, with higher than 43.36 wt.%, and potassium sulphate is another form of alkali metal. However, the main form of alkali metal in the SEH ash is silicate, and the element Cl in straw ash is totally bound to K but is not in SEH ash. For low-chlorine wood chip ash, the element K exists in sulphate and aluminosilicate, while aluminosilicate is the domain form of alkali metal in low-chlorine DNH ash. For low-Cl coal ash and biomass ash, the main composition is quartz and calcium salt, but the proportion of quartz in DNH ash is higher than that in wood chip ash. After exposure, the mass gain of three steel specimens corroded in two biomass ashes is shown in Figure 8. It is noted that the mass gain of steel specimens corroded in straw ash is negative; that is, the corrosion product is loose and peeled-off scale. Although alloy TP347H has a high content of Cr and Ni, a large amount of oxide scale is detached from the substrate because of the higher content of KCl in the straw ash. Corrosive KCl would evaporate in the flue gas temperature and condense in the relatively cold metal surface; that is, the crystal KCl can be rich in the inner deposit, similar to the NaCl in the SEH ash deposit. Gaseous KCl can also react with protective oxide scale Cr2O3 to form 11


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unprotective alkali chromate, releasing Cl2 or HCl, as shown in equations 1 and 2. 4KCl(g) + Crଶ Oଷ (s) + 2.5Oଶ (g) ↔ 2Kଶ CrOସ (s) + 2Clଶ (g)

4NaCl(g) + Crଶ Oଷ (s) + 2.5Oଶ (g) ↔ 2Naଶ CrOସ (s) + 2Clଶ (g) where △G represents the Gibbs free energy at 610

△G=-212 kJ/mol

(1)

△G=-222 kJ/mol

(2)

, and the negative value indicates that the

reaction is thermodynamically favoured. Figure 8 shows that the mass gain values of low-Cl wood chip ash are slightly higher than those of DNH ash because alkali metal sulphate exists in the wood chip ash. K2SO4 can react with iron oxide to form alkali metal trisulphate, but it cannot react with Cr or Cr2O3, in agreement with the work by Paneru et al.20 The protective oxide scale Cr2O3 forms on the surface of alloy TP347H because of the higher Cr content than in the others; thus, the least amount of mass gain is achieved for alloy TP347H. Figure 9 displays the cross-sectional microstructure and elemental distribution map results of the corrosion products of TP347H steel at 610

corroded in straw ash deposit for 168 h. It can be

observed that an obvious crack exists in the metal substrate, and element Cl is not detected in the maps. According to the NIST Chemistry standard database, NaCl and KCl saturated vapour pressure can be calculated as follows: log൫pୱୟ୲,୒ୟେ୪ ൯ = 3.56682 − (

ହଶ଴଴.ଽ଴ସ

୘ିସସ.ଶହଽ

଻ସସ଴.଺ଽଵ

log൫pୱୟ୲,୏େ୪ ൯ = 4.78236 − (୘ାଵହ଴.ସସଵ)

)

(3)

(4)

where P is the vapor pressure of gaseous alkali metal chlorides (bar), and T is temperature ( ). Figure 10 gives the curves of the saturated vapor concentration of alkali chlorides versus temperature according to equations 3 and 4; the saturated vapor concentration of NaCl and KCl is found to reach up to 10800 ppm and 6600 ppm at a flue gas temperature of 950 20 ppm and 8 ppm at the metal surface temperature of 610

, respectively, but the values are

. The difference in vapor concentration

12


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at the flue gas temperature and the metal surface temperature lead to the formation of the evaporation-condensation mechanism.16 It can also be seen that vapour pressure of KCl is higher than that of NaCl at the same temperature. Thus, a higher amount of Cl2 or HCl is released from the reaction with protective oxide scale corroded in straw ash than that in SEH ash. The released chlorine can penetrate the substrate through the pores and cracks, reacting with alloying elements to form metal chlorides at the scale-metal interface. The formed chlorides can be thermodynamically stable where the oxygen is very low, and high vapor pressure can guarantee continuous diffuse outward to the scale surface.27 Oxygen concentration increases during the outward movement, and the oxidation of chlorides to non-protective metal oxides occur, releasing Cl2 without net consumption.1 The total process is called active oxidation corrosion,6 as shown in equations 5-7. M(s) + Clଶ (g) ↔ MClଶ(s)

(5)

MClଶ (s) ↔ MClଶ (g)

(6)

2MClଶ (g) + Oଶ (g) ↔ Mଶ Oଷ (s) + 2Clଶ (g)

(7)

where M represents Fe, Cr, and Ni. As aforementioned, alkali chromate Na2CrO4 can combine with NaCl to form low-temperature eutectic, and the melting temperature is 557

, which causes the

molten phase to occur at the alloy/scale interface. However, the melting temperature of KCl-K2CrO4 is 650

;28 that is, no obvious molten phase occurs at the metal temperature 610

. A large amount

of oxide scale forms on the substrate because of the high concentration of KCl (g), and the corrosion product is detached from the substrate. Thus, the corrosion mechanism of straw ash can be inferred by high-temperature active oxidation, where Cl2 is assumed as a catalyst, and for SEH ash, accelerated corrosion is the complicated type of high-temperature molten salts. The corrosion 13


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reaction rate in the liquid phase is faster than in solid-solid reactions, and electrochemical attack can occur.4

3.3 Chlorine corrosion during co-firing of coal and biomass From the perspective of practical application in the utility boiler, co-firing of reasonable coal and biomass is an effective and economical approach to increase the utilization of biofuel and decrease the pollutant emission.29 In view of alleviating high-temperature corrosion, it is meaningful to investigate the characteristic of high-Cl coal ash mixed with low-Cl biomass ash or low-Cl coal ash mixed with high-Cl biomass ash. Many elements are recognized in the literature30 to contribute to high-temperature corrosion in utility boiler-firing coal or biomass, such as Cl, S, Na, K, Si, Al, O, Ca, Mg and C, various chemical forms with chloride, sulphate, silicate, oxide, hydroxide, carbonate, and aluminosilicate. Table 3 lists the physical properties of common alkali metal salts in deposit on boiler tube surface, and it is generally accepted to translate the alkali chlorides and sulphates into aluminosilicate for mitigating attack.4 NaCl and KCl are common chlorides found in deposits formed during coal and biomass combustion, respectively.31 Compared to alkali chlorides, the corrosivity of alkali sulphates is less severe because of the higher melting point of alkali sulphates, and an alloy rich in Cr and Ni is capable of withstanding S-induced corrosion.20

After exposure, the mass gain of three steel specimens corroded in two mixed ashes is shown in Figure 11. It is noted that the mass gain of T91 and 12CrMoVG are still high because the protective oxide scale is not totally formed on the alloy surface with low content of Cr and Ni. The focus is on the alloy TP347H to consider the effectiveness of co-firing on alleviating high-temperature corrosion. For a mixture of DNH and straw ash deposit, the value of mass gain is approximately 2 mg/cm2, and for a mixture of SEH and wood chip ash, the value is approximately 6 mg/cm2, which is close to that 14


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SEH ash. That is, for high-Cl biomass ash, it is effective to reduce corrosion when DNH ash is added at the weight ratio of 2 to 1. However, for high-Cl coal ash, the effect of adding low-Cl wood chip ash is not obvious. As potassium is a soluble form of chloride or sulphate in straw ash,32 it can be inferred that this potassium can react with SiO2 and Al2O3 to form silicates or aluminosilicates through silication and alumina-silication with the addition of an adequate amount of DNH ash, releasing gaseous HCl. As seen in equations 8-12, the Gibbs free energy of reactions between gaseous alkali chloride and SiO2 is negative and thermodynamically favoured, but alkali sulphate could not be captured to form aluminosilicate. Because of the high amount of calcium oxide in DNH ash, the corrosive sulphate can be bind with CaO to form calcium sulphate. In Section 3.2, the high temperature corrosion of steel specimens corroded in straw ash is mainly caused by high concentration of gaseous KCl close to the metal surface, reacting with protective oxide scale Cr2O3 and releasing a large amount of chlorine into the substrate. In the mixture of straw ash and DNH ash, the corrosive KCl(g) is not available on the tube surface; thus, the effect of alleviating attack is obvious in the straw ash. 2KCl(g) + ‫ݔ‬SiOଶ (s) + Hଶ O(g) ↔ K ଶ O ∗ ‫ݔ‬SiOଶ (s, l) + 2HCl(g)

△G=-33 kJ/mol

(8)

2NaCl(g) + ‫ݔ‬SiOଶ (s) + Hଶ O(g) ↔ Naଶ O ∗ ‫ݔ‬SiOଶ (s, l) + 2HCl(g) △G=-68 kJ/mol

(9)

2KCl(s) + Alଶ Oଷ ∗ ‫ݔ‬SiOଶ (s) + Hଶ O(g) ↔ K ଶ O ∗ Alଶ Oଷ ∗ ‫ݔ‬SiOଶ (s) + 2HCl(g) △G=-72 kJ/mol (10) 4NaCl(s) + Alଶ Oଷ ∗ ‫ݔ‬SiOଶ (s) + 2HଶO(g) ↔ Naଶ O ∗ Alଶ Oଷ + Naଶ O ∗ ‫ݔ‬SiOଶ (s) + 4HCl(g) △G=-157 kJ/mol (11) K ଶ SOସ (s) + Alଶ Oଷ ∗ ‫ݔ‬SiOଶ (s) + Hଶ O(g) ↔ K ଶ O ∗ Alଶ Oଷ ∗ ‫ݔ‬SiOଶ (s) + Hଶ SOସ (g) △G=+19 kJ/mol (12)

For the mixture of SEH ash and wood chip ash, the mass gain of TP347H is close to the value of SEH ash deposit. As aforementioned in Section 3.1, the corrosion of the specimen corroded in SEH ash results from two pathways: (1) the reaction of NaCl(g) with Cr2O3 can destroy protective oxide 15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

scale, releasing chlorine into the substrate to accelerate oxidation, and (2) the formed yellow alkali chromate can combine with NaCl to result in low-temperature eutectic to flux the oxide scale. Moreover, ionic charge transfer for electrochemical attack occurs in the molten phase.4 With the addition of wood chip ash at the mass ratio 1:2 into SEH ash, the amount of quartz is not high enough to capture NaCl, and alkali sulphate can form the low-melting eutectic with NaCl.33 Moreover, NaCl(g) in the SEH ash can replace K in the potassium aluminosilicate,1 releasing KCl(g), as shown in equation 13. Furthermore, KCl can form the low-melting eutectic with NaCl once the local concentration reaches the eutectic point. Thus, for high-Cl SEH ash, by adding wood chip ash at the ratio of 2:1, it can be inferred that a molten phase may exist at the substrate/scale interface, implying catastrophic corrosion because the rate of molten corrosion is much higher than the rate of chemical corrosion.4 2NaCl(g) + ݉K ଶ O ∗ ‫ݕ‬Alଶ Oଷ ∗ ‫ݔ‬SiOଶ (fused) ↔ Naଶ O ∗ (݉−1)K ଶ O ∗ ‫ݕ‬Alଶ Oଷ ∗ ‫ݔ‬SiOଶ (fused) + 2KCl(g) (13)

Figure 12 shows the cross-sectional microstructures of TP347H corroded in two blended ashes and the compact oxide layer formed on the surface of TP347H corroded under blended ash deposit of DNH and straw. A large amount of corrosion product is formed on the metal surface corroded under blended ash deposit of SEH and wood chip ashes, as reflected in the value of mass gain. Figure 13 displays the cross-sectional microstructure and elemental distribution map results of TP347H steel at 610

corroded in blended ash deposit of DNH and straw ashes for 168 h. It can be observed that

protective oxide scale is not destroyed and that no corrosive elements are detected in the substrate; i.e., high-temperature corrosion induced by high Cl straw ash can be alleviated by the addition of DNH ash at the weight ratio of 2 to 1.

4. CONCLUSIONS 16


Page 16 of 32

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Energy & Fuels

The effects of firing high-Cl coal, high-Cl biomass, and co-firing of coal/biomass on the fireside corrosion of superheater/reheater components are significant issues for utility boilers. This paper reports the results of a series of high temperature corrosion experiments conducted at metal temperature of 610

and flue gas temperature of 950

for 168 h using the double-tube corrosion

probe to regulate the two different temperatures. The steel specimens are more susceptible to be destroyed under high-Cl SEH ash or straw ash deposit, as evidenced by greater weight gains. In addition, austenitic steel TP347H, characterized with high contents of Cr and Ni, exhibits higher corrosion resistance than that of other specimens. The corrosion mechanism of straw ash can be inferred by high-temperature active oxidation due to the high vapor concentration of gaseous KCl, and the formed oxide scale is loose and easily detached from the substrate. The corrosion mechanism of SEH ash is the complicated type of high-temperature molten salts. The low-melting eutectic of NaCl-Na2CrO4 can flux the protective oxide scale at the metal temperature of 610

.

With the addition of DNH ash into the corrosive straw ash, the high-temperature corrosion could be reduced because a large amount of Si, Al, and Ca are provided from DNH ash to capture K and S in the straw ash; thus, the vapor concentration of gaseous KCl close to the metal surface decreases. With the addition of wood chip ash into the corrosive SEH ash, the effect of alleviating corrosion is insignificant at the mass ratio of coal ash and biomass ash of 2:1. The silication and alumina-silication of NaCl are not complete because the element Si in wood chip ash is not high 17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

enough, and there exists K2SO4 in the wood chip ash to promote the attack with NaCl in the SEH ash.

AUTHOR INFORMATION Corresponding Author *Telephone: +86-21-34206049. Fax: +86-21-34206115. E-mail address: [email protected]. Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENTS: This work was supported by the social development project of the Shanghai Science and Technology Commission (Grant NO. 15DZ1200600). We thank the Instrumental Analysis Center of Shanghai Jiao Tong University (SJTU) for providing the assistance of SEM-EDS, XRF, and XRD instruments.

REFERENCES (1) Tillman, D. A.; Duong, D.; Miller, B. Chlorine in Solid Fuels Fired in Pulverized Fuel Boilers s Sources, Forms, Reactions, and Consequences: a Literature Review. Energy & Fuels 2009, 23, 3379-3391. (2) Silva, R. B.; Fragoso, R.; Sanches, C.; Costa, M.; Martins-Dias, S. Which chlorine ions are currently being quantified as total chlorine on solid alternative fuels? Fuel Processing Technology 2014, 128, 61-67. (3) Wei, X.; Lopez, C.; Puttkamer, T. v.; Schnell, U.; Unterberger, S.; Hein, K. R. G. Assessment of Chlorine-Alkali-Mineral Interactions during Co-Combustion of Coal and Straw. Energy & Fuels 2002, 16, 1095-1108.

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Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(4) Niu, Y.; Tan, H.; Hui, S. e. Ash-related issues during biomass combustion: Alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Progress in Energy and Combustion Science 2016, 52, 1-61. (5) Enestam, S.; Bankiewicz, D.; Tuiremo, J.; Mäkelä, K.; Hupa, M. Are NaCl and KCl equally corrosive on superheater materials of steam boilers? Fuel 2013, 104, 294-306. (6) Antunes, R. A.; de Oliveira, M. C. L. Corrosion in biomass combustion: A materials selection analysis and its interaction with corrosion mechanisms and mitigation strategies. Corrosion Science 2013, 76, 6-26. (7) Bala, N.; Singh, H.; Prakash, S. Accelerated hot corrosion studies of cold spray Ni–50Cr coating on boiler steels. Materials & Design 2010, 31 (1), 244-253. (8) Ja'baz, I.; Chen, J.; Etschmann, B.; Ninomiya, Y.; Zhang, L. High-temperature tube corrosion upon the interaction with Victorian brown coal fly ash under the oxy-fuel combustion condition. Proceedings of the Combustion Institute 2017, 36 (3), 3941-3948. (9) Schofield, K. A New Method to Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. I. Tungsten Salts. Energy & Fuels 2003, 17, 191-203. (10) Schofield, K. New Method To Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. II. Molybdenum Salts. Energy & Fuels 2005, 19, 1898-1905. (11) Kassman, H.; Pettersson, J.; Steenari, B.-M.; Åmand, L.-E. Two strategies to reduce gaseous KCl and chlorine in deposits during biomass combustion — injection of ammonium sulphate and co-combustion with peat. Fuel Processing Technology 2013, 105, 170-180. (12) Khalil, R. A.; Houshfar, E.; Musinguzi, W.; Becidan, M. l.; Skreiberg, Ø.; Goile, F.; Løvås, T.; Sørum, L. Experimental Investigation on Corrosion Abatement in Straw Combustion by Fuel Mixing. Energy & Fuels 2011, 25 (6), 2687-2695.

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(13) Lith, S. C. v.; Frandsen, F. J.; Montgomery, M.; Vilhelmsen, T.; Jensen, S. A. Lab-scale Investigation of Deposit-induced Chlorine Corrosion of Superheater Materials under Simulated Biomass-firing Conditions. Part 1: Exposure at 560 °C. Energy & Fuels 2009, 23, 3457-3468. (14) Stein-Brzozowska, G.; Maier, J.; Scheffknecht, G. Impact of the oxy-fuel combustion on the corrosion behavior of advanced austenitic superheater materials. Energy Procedia 2011, 4, 2035-2042. (15) Zhan, Z.; Bool, L. E.; Fry, A.; Fan, W.; Xu, M.; Yu, D.; Wendt, J. O. L. Novel Temperature-Controlled Ash Deposition Probe System and Its Application to Oxy-coal Combustion with 50% Inlet O2. Energy & Fuels 2014, 28 (1), 146-154. (16) Lindberg, D.; Niemi, J.; Engblom, M.; Yrjas, P.; Laurén, T.; Hupa, M. Effect of temperature gradient on composition and morphology of synthetic chlorine-containing biomass boiler deposits. Fuel Processing Technology 2016, 141, 285-298. (17) Kumar, M.; Singh, H.; Singh, N.; Joshi, R. S. Erosion–corrosion behavior of cold-spray nanostructured Ni–20Cr coatings in actual boiler environment. Wear 2015, 332-333, 1035-1043. (18) Liu, Y.; Fan, W.; Zhang, X.; Wu, X. High-Temperature Corrosion Properties of Boiler Steels under a Simulated High-Chlorine Coal-Firing Atmosphere. Energy & Fuels 2017, 31 (4), 4391-4399. (19) Zhou, B.; Zhou, H.; Wang, J.; Cen, K. Effect of temperature on the sintering behavior of Zhundong coal ash in oxy-fuel combustion atmosphere. Fuel 2015, 150, 526-537. (20) Paneru, M.; Stein-Brzozowska, G.; Maier, J.; Scheffknecht, G. Corrosion Mechanism of Alloy 310 Austenitic Steel beneath NaCl Deposit under Varying SO2Concentrations in an Oxy-fuel Combustion Atmosphere. Energy & Fuels 2013, 27 (10), 5699-5705.

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Energy & Fuels

(21) Miltner, A.; Beckmann, G.; Friedl, A. Preventing the chlorine-induced high temperature corrosion in power boilers without loss of electrical efficiency in steam cycles. Applied Thermal Engineering 2006, 26 (16), 2005-2011. (22) Lehmusto, J.; Skrifvars, B. J.; Yrjas, P.; Hupa, M. High temperature oxidation of metallic chromium exposed to eight different metal chlorides. Corrosion Science 2011, 53 (10), 3315-3323. (23) Ja'baz, I.; Jiao, F.; Wu, X.; Yu, D.; Ninomiya, Y.; Zhang, L. Influence of gaseous SO 2 and sulphate-bearing ash deposits on the high-temperature corrosion of heat exchanger tube during oxy-fuel combustion. Fuel Processing Technology 2017, 167, 193-204. (24) Bankiewicz, D.; Vainikka, P.; Lindberg, D.; Frantsi, A.; Silvennoinen, J.; Yrjas, P.; Hupa, M. High temperature corrosion of boiler waterwalls induced by chlorides and bromides – Part 2: Lab-scale corrosion tests and thermodynamic equilibrium modeling of ash and gaseous species. Fuel 2012, 94, 240-250. (25) Lehmusto, J.; Lindberg, D.; Yrjas, P.; Skrifvars, B. J.; Hupa, M. Thermogravimetric studies of high temperature reactions between potassium salts and chromium. Corrosion Science 2012, 59, 55-62. (26) Li, L.; Duan, Y.; Cao, Y.; Chu, P.; Carty, R.; Pan, W.-P. Field corrosion tests for a low chromium steel carried out at superheater area of a utility boiler with three coals containing different chlorine contents. Fuel Processing Technology 2007, 88 (4), 387-392. (27) Uusitalo, M. A.; Vuoristo, P. M. J.; Mäntylä, T. A. High temperature corrosion of coatings and boiler steels below chlorine-containing salt deposits. Corrosion Science 2004, 46 (6), 1311-1331. (28) Janz, G. J.; A1len, C. B.; Downey, J.; Tamkins, R. Eutectic data: safety, hazard, corrosion, melting points, compositions and bibliography. Troy, NY: Molten Salts Data Center, Rensselaer Polytechnic Institute, 1976. (29) Aho, M.; Silvennoinen, J. Preventing chlorine deposition on heat transfer surfaces with aluminium–silicon rich biomass residue and additive. Fuel 2004, 83 (10), 1299-1305.

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(30) Aho, M.; Ferrer, E. Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass. Fuel 2005, 84 (2-3), 201-212. (31) Schofield, K. The chemical nature of combustion deposition and corrosion: The case of alkali chlorides. Combustion and Flame 2012, 159 (5), 1987-1996. (32) Pisa, I.; Lazaroiu, G. Influence of co-combustion of coal/biomass on the corrosion. Fuel Processing Technology 2012, 104, 356-364. (33) Wu, X.; Zhang, X.; Dai, B.; Xu, X.; Zhang, J.; Zhang, L. Ash deposition behaviours upon the combustion of low-rank coal blends in a 3 MW th pilot-scale pulverised coal-fired furnace. Fuel Processing Technology 2016, 152, 176-182.

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Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 1. Schematic diagram for lifecycle of element Cl in coal combustion.2

Figure 2. Experimental setup for the corrosion tests. 23


Energy & Fuels

b

a:Sodium iron Oxide-NaO⋅FeO

Wt.% 0.6

b:Sodium Calcium Silicate-2Na SiO ⋅CaSiO 2 3 3

20.01

c:Langbeinite-K2SO4⋅2MgSO4

17.88

d:Calcium Sodium Aluminium Oxide-8CaO⋅NaO⋅3Al O 2 3

11.38 0.53

Intensity (a.u.)

e:Halite-NaCl

10.64

f:Quartz-SiO2

a d

g:Srebrodolskite-CaFeO

9.18 4

2.83

b h:Aluminum Nitride-AlN

c g

10

d

k

20

5.67

k:Sodium Silicate-2Na SiO 2 3

j ccf f

8.42

j:poly (o-methylaniline) perchlorate-(C28H28Cl2N4O8)n

c

e

30

40

d

h

d

f

d

50

d

f

bc

60

b

f

70

80

90

2θ (°)

(a)

Wt.% 41.3

a:Quartz-SiO2

a

15.93

b:Calcite-CaCO3

7.35

c:Calcium Oxide-CaO

15.74

d:Calcium Sulfate-CaSO4

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

e:Iron Sulfide-Fe3S

2.12

f:Muscovite-K2O⋅Al2O3⋅6SiO2⋅2H2O

3.84

g:Analcime-5Na2O⋅3MgO⋅8Al2O3⋅32SiO2⋅25H2O

9.6

d a b g

10

f

20

d

b

30

ca

a e b c c

40

50

a a a b

60

a

70

a a a

80

90

2θ (°) (b)

Figure 3. XRD analysis of coal ash prepared at 500 ℃: (a) SEH ash, (b) DNH ash. 24


Page 25 of 32

50

12Cr1MoVG T91 TP347H

2

40

Mass gain,mg/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

30

20

10

TP347H SEH ash deposit

0

SEH ash

DNH ash

Figure 4. Mass gain of the steel specimens corroded in various coal ash deposits.

BSE

Fe

Cr

Ni

O

Na

Cl

Si

Ca

S

Figure 5. Cross-sectional microstructure and elemental distribution maps of TP347H steel corroded in SEH ash.

25


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

Type 1 oxide

T91 SEH ash deposit

12Cr1MoVG SEH ash deposit

(a)

(b)

TP347H DNH ash deposit

(c)

Figure 6. Cross-sectional microstructure of specimen corroded in two coal ash deposits.

26


Page 27 of 32

Wt.% a

a:Sylvite-KCl

43.36

b:Arcanite-K2SO4

15.47

c:Calcite-CaCO3

15.86

d:Calcium Oxide-CaO 10.42 4.46

Intensity (a.u.)

e:Quartz-SiO2

a

b d

10

a

bb e c

a b

b

20

30

c

40

50

c

a

a

c

60

70

80

90

2θ (°) (a) Wt.% 41.3

a:Quartz-SiO2

15.93

b:Calcite-CaCO3

b

7.35

c:Calcium Oxide-CaO

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

d:Calcium Sulfate-CaSO4

15.74

e:Potassium Sulfate-K2SO4

2.12

f:Muscovite-K2O⋅Al2O3⋅6SiO2⋅2H2O

3.84 9.6

a c

f

10

b d

20

e

e

a a b f

e

d e

30

cba

b

40

50

60

f

a

70

a a

80

90

2θ (°) (b)

Figure 7. XRD analysis of biomass ash prepared at 500 ℃: (a) straw ash, (b) wood chip ash. 27


Energy & Fuels

-50

50


40

2

-40

Mass gain,mg/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

-30

30

-20

20

TP347H Straw ash deposit -10

10

0

0

Straw ash

Wood chip ash

Figure 8. Mass gain of the steel specimens corroded in various biomass ash deposits.

BSE

Fe

Cr

Ni

O

Si

K

S

Figure 9. Cross-sectional microstructure and elemental distribution maps of TP347H steel corroded in straw ash.

28


Page 29 of 32

1000 150

Saturation vapor concentration,ppm

800

100 50

600

400

0 -50 500

550

600

650

700

200

NaCl KCl 0 400

500

600

700

800

900

1000

o

Temperature, C

Figure 10. Saturation vapor concentration of alkali chlorides versus temperature.

50


40

Mass gain,mg/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

30

20

10

0

DNH+Straw ash SEH+Wood chip ash

Figure 11. Mass gain of the steel specimens corroded in various blended ash deposits.

29


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TP347H SEH +Wood chip ash deposit

Page 30 of 32

TP347H DNH +Straw ash deposit

Figure 12. Cross-sectional microstructure of a specimen corroded in two different blended ash deposits.

BSE

Fe

Cr

Ni

O

Al

Mn

Figure 13. Cross-sectional microstructure and elemental distribution maps of TP347H steel corroded in blended ash deposit of DNH and straw.

30


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Energy & Fuels

Table 1. Compositions of the four high-alkali coals with various chlorine contents. Coal

Biomass

SEH

DNH

Straw

Wood chips

Moisture (wt.%, ad)

23.93

20.84

10.87

7.42

Ash content (wt.%, ar)

8.05

17.2

9.34

1.13

C

50.82

66.81

39.21

45.03

H

2.16

2.57

4.1

8.13

O

14.9

11.35

34.8

36.82

N

0.73

0.78

0.46

1.36

S

0.23

0.93

0.26

0.014

Si

7.67

19.95

17.54

8.56

Fe

4.35

7.88

0.49

2.64

Al

5.71

11.39

0.43

2.19

Ca

30.53

13.78

5.27

26.15

Mg

1.88

2.99

1.21

3.75

K

0.17

1.25

30.25

12.54

Na

4.63

2.55

0.43

1.32

Ti

0.54

0.71

0.05

0.37

S

1.45

6.49

2.80

1.22

Mn

0.07

0.09

0.04

0.28

O

15.62

31.45

30.11

38.94

Cl

11.37

0.70

10.29

0.22

Ultimate analysis (wt.%, ar)

Chemical composition of ash (wt.%, dry basis @ 500

)

31


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

Table 2. Laboratory corrosion testing. Steel type

T91, 12Cr1MoVG, TP347H

Simulated flue gas flow rate

100 ml/min

Flue gas temperature

950 ℃

Corrosion time

168 h

Metal temperature

610 ℃

Deposit

SEH ash, DNH ash, Straw ash, Wood chip ash, SEH ash+ Wood chip ash, DNH ash+ Straw ash

Simulated flue gas compositions

4% O2+16% CO2+9% H2O+N2

Table 3. Comparison of physical properties of alkali metal salts in deposit on heating surface.

Alkali metal salt

Chemical formula

chloride

MCl

★

Yes

★★★★

sulphate

M2SO4

★★

Yes

★★

silicate

M2O﹒SiO2

★★★

No

No

aluminosilicate

M2O﹒Al2O3﹒SiO2

★★★★

No

No

Melting point* Deactivation of SCR catalyst

*a higher number of “★” represents higher melting point and corrosivity.

32


Corrosivity*

Chlorine-Induced High-Temperature Corrosion of Boiler Steels

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